Method for Retransmitting a Data Sequence According to Channel Condition in a Wireless Communications Network

ABSTRACT

The present invention discloses a method for retransmitting a data sequence according to channel condition in a wireless communications network. The method comprises transmitting a first transmitting signal representing the data sequence using a first set of beamforming weighting vectors generated according to a first channel condition, receiving a request for re-transmitting the data sequence, wherein the first transmitting signal suffers from unrecoverable errors, computing a re-transmission function according to a second channel condition, and transmitting a second transmitting signal generated by using the re-transmission function that is created according to the second channel condition, wherein the data sequence is demodulated and decoded using the first transmitting and the second transmitting signals.

CROSS REFERENCE

The present application claims the benefit of U.S. ProvisionalApplication Ser. 60/897,759, which was filed on Jan. 26, 2007.

BACKGROUND

A typical multiple-input-multiple-output (MIMO) network comprises a basetransceiver station (BTS) with an antenna array and multiple mobilestations (MSs), at least one of which has multiple antennas. It has beendemonstrated that employing a beamforming technique can enhance theperformance of an MIMO network. Therefore, the beamforming technique hasbeen adopted by several wireless communications standards, such as IEEE802.16 d/e (WiMAX)

In an MIMO network employing a beamforming technique, a BTS and an MSrely on beamformed signals to communicate with each other. The signalstransmitted from the multiple antennas on the BTS are weighted based onphase and magnitude. The BTS computes beamforming weighting vectors fora receiver of a wireless communications network according to channelcondition. Subsequently, the beamforming weighting vectors are appliedto the multiple antennas on the BTS to de-correlate transmitting signalson the beamformed channels.

The performance of a wireless communications network is often evaluatedbased on its capacity and throughput. One of the factors that impactnetwork performance is that the transmitter of a message has the exactinformation about the channel condition between the transmitter and thereceiver. More specifically, whether employing a beamforming techniquewill result in optimal network performance depends on the accuracy ofthe channel condition that the transmitter obtains.

Although the combination of beamforming and MIMO techniques furtherimproves network performance, the data transmitted via a wirelesschannel may still be corrupted due to unexpected impairment of thechannel condition. To deal with this issue, a technique ofre-transmission, e.g. automatic repeat request (ARQ), is used. The ARQis a conventional scheme in which a wireless receiver requestsre-transmission of a data sequence when unrecoverable frame errors aredetected at the receiving end. The most commonly used error detectingcode is the cyclic redundancy check (CRC) code.

A data sequence can be protected by an error correcting code, whichincreases the probability of a successful transmission. An ARQ schemethat combines the ARQ principle with error correcting code is known as ahybrid ARQ (HARQ) scheme. In a conventional HARQ scheme, there-transmitted data sequence is encoded exactly the same way as thefirst transmission.

The wireless receiver combines the re-transmitted data sequence with thepreviously received one and then decodes the combined data sequence. Asa result, the re-transmitted data sequence, which is subject to the samechannel condition as the original transmission, suffers from the sametype of unrecoverable error. The conventional HARQ algorithm onlyprovides limited improvement to the performance of the system.

As such what is desired is method for re-transmitting a data sequenceusing a different encoding method to avoid unrecoverable errors in awireless communications network.

SUMMARY

The present invention discloses a method for retransmitting a datasequence according to channel condition in a wireless communicationsnetwork. The method comprises transmitting a first transmitting signalrepresenting the data sequence using a first set of beamformingweighting vectors generated according to a first channel condition,receiving a request for re-transmitting the data sequence, wherein thefirst transmitting signal suffers from unrecoverable errors, computing are-transmission function according to a second channel condition, andtransmitting a second transmitting signal generated by using there-transmission function that is created according to the second channelcondition, wherein the data sequence is demodulated and decoded usingthe first transmitting and the second transmitting signals.

The construction and method of operation of the invention, however,together with additional objects and advantages thereof, will be bestunderstood from the following description of specific embodiments whenread in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING

The drawings accompanying and forming part of this specification areincluded to depict certain aspects of the invention. The invention maybe better understood by reference to one or more of these drawings incombination with the description presented herein. It should be notedthat the features illustrated in the drawings are not necessarily drawnto scale.

FIG. 1 shows an 8×2 MIMO system comprising a base transceiver stationand a mobile station.

FIG. 2 is a flow diagram illustrating the method disclosed in thepresent invention.

DESCRIPTION

The following detailed description of the invention refers to theaccompanying drawings. The description includes exemplary embodiments,not excluding other embodiments, and changes may be made to theembodiments described without departing from the spirit and scope of theinvention. The following detailed description does not limit theinvention. Instead, the scope of the invention is defined by theappended claims.

The present invention discloses a method for retransmitting a datasequence using a different encoding method to avoid unrecoverable errorsin a wireless communications network. The disclosed method is applicableto an M×N multiple-input-multiple-output (MIMO) network employing abeamforming technique. An exemplary 8×2 MIMO system is presented forpurposes of illustrating the present invention.

A data sequence is encoded and modulated into a sequence of OrthogonalFrequency-Division Multiplexing (OFDM) symbols. The sequence of OFDMsymbols is further divided into a plurality of transmission unitscomprising two or more OFDM symbols. Two or more OFDM symbols in atransmission unit are combined according to a predetermined rule to forma transmitting signal transmitted by an antenna array.

FIG. 1 shows an 8×2 MIMO system 100 comprising a base transceiverstation (BTS) 110 and a mobile station (MS) 120. The BTS 110 is equippedwith an antenna array 115 of eight antennas while the MS 120 is equippedwith an antenna array 125 of two antennas.

The BTS 110 has a set of beamforming weighting vectors {right arrow over(w)}₁ and {right arrow over (w)}₂ corresponding to the two antennas ofthe antenna array 125 on the MS, where {right arrow over(w)}_(i)=(w_(i1), w_(i2), . . . , w_(i8))^(H); i={1,2}; and ( . . .)^(H) is a Hermitian operator. In one embodiment of the presentinvention, a transmitting signal {right arrow over (s)} 130 is generatedby the BTS 110 according to the following equation: {right arrow over(s)}=x₁{right arrow over (w)}₁+x₂{right arrow over (w)}₂, where (x₁, x₂)is a transmission unit of OFDM symbols.

Let {right arrow over (h)}₁ and {right arrow over (h)}₂ be a set ofchannel response functions representing the channel condition betweenthe antenna array 115 and antenna array 125, where h_(i) =(h_(i1),h_(i2), . . . , h_(i8))^(H); i={1,2}; h_(ij) is the channel responsefunction between antenna i of the antenna array 125 and antenna j of theantenna array 115; and ( . . . )^(H) is a Hermitian operator.

The signals received by the antennas of the antenna array 125 arerepresented by the following equations: r₁={right arrow over (h)}₁^(H){right arrow over (w)}₁x₁+{right arrow over (h)}₁ ^(H){right arrowover (w)}₂x₂ (1) and r₂={right arrow over (h)}₂ ^(H){right arrow over(w)}₁x₁+{right arrow over (h)}₂ ^(H){right arrow over (w)}₂x₂ (2), wherer₁ is the signal received by the first antenna of the antenna array 125and r₂ is the signal received by the second antenna.

The beamforming technique described above effectively creates a virtualchannel between the BTS and the MS. The virtual channel is described bythe following matrix:

${\begin{matrix}{{\overset{\rightarrow}{h}}_{1}{\overset{\rightarrow}{w}}_{1}} & {{\overset{\rightarrow}{h}}_{1}{\overset{\rightarrow}{w}}_{2}} \\{{\overset{\rightarrow}{h}}_{2}{\overset{\rightarrow}{w}}_{1}} & {{\overset{\rightarrow}{h}}_{2}{\overset{\rightarrow}{w}}_{2}}\end{matrix}}\quad$

(3). In order to obtain optimal performance of the MIMO wirelessnetwork, the set of beamforming weighting vectors {right arrow over(w)}₁ and {right arrow over (w)}₂ is generated in such a way that thevalues of {right arrow over (h)}₁ ^(H){right arrow over (w)}₂ and {rightarrow over (h)}₂ ^(H){right arrow over (w)}₁ are as small as possible,and the values of {right arrow over (h)}₁ ^(H){right arrow over (w)}₁and {right arrow over (h)}₂ ^(H){right arrow over (w)}₂ are as large aspossible.

The method disclosed in the present invention presents a novel way tore-transmit the data sequence by applying a re-transmission function toa transmission unit to generate a re-transmitting signal {right arrowover (s)}′. The re-transmission function is generated based on channelcondition. The re-transmitting signal is combined with the previoustransmitting signal to increase the diversity of the signal received bythe wireless receiver, which in turn increases the probability that thesignal is successfully demodulated and decoded.

FIG. 2 is a flow diagram illustrating the method disclosed in thepresent invention. A transmitting signal is generated from atransmission unit of OFDM symbols according to the method describedabove, and it is sent from a BTS to an MS (step 210). If thetransmitting signal is corrupted, the BTS will receive a negativeacknowledgement from the MS regarding unrecoverable errors in thetransmitting signal (step 220). In step 230, the BTS computes are-transmission function according to the current channel condition.Subsequently, the re-transmission function is applied to thetransmission unit and thus a re-transmitting signal is generated. Instep 240, the re-transmitting signal is generated and sent from the BTSto the MS.

In one embodiment of the present invention, the channel conditionbetween the BTS and the MS is quasi-static. In other words, the channelcondition remains the same during the first transmission and there-transmission of the data sequence. The re-transmission function,defined as

${{f\begin{pmatrix}x_{1} \\x_{2}\end{pmatrix}} = \begin{pmatrix}{- x_{2}^{H}} \\x_{1}^{H}\end{pmatrix}},$

is applied to the transmission unit. Subsequently, a re-transmittingsignal {right arrow over (s)}′, defined as {right arrow over (s)}′=(−x₂^(H)){right arrow over (w)}₁+(x₁ ^(H)){right arrow over (w)}₂, isgenerated. The re-transmitting signal is orthogonal to the transmittingsignal.

The signals received by the antennas of the antenna array arerepresented by the following equations: r₁′={right arrow over (h)}₁^(H){right arrow over (w)}₁(−x₂ ^(H))+{right arrow over (h)}₁ ^(H){rightarrow over (w)}₂(x₁ ^(H)) (4) and r₂′={right arrow over (h)}₂ ^(H){rightarrow over (w)}₁(−x₂ ^(H))+{right arrow over (h)}₂ ^(H){right arrow over(w)}₂(x₁ ^(H)) (5), where r′₁ is the signal received by the firstantenna of the antenna array and r′₂ is the signal received by thesecond antenna.

Because the channel condition is quasi-static, the beamforming weightingvector {right arrow over (w)}_(i) for the re-transmitting signal is thesame as that for the transmitting signal. This is also true for thechannel response function {right arrow over (h)}_(i). Because thetransmitting and re-transmitting signals are orthogonal to each other,the disclosed method creates spatial diversity.

A simplified maximum likelihood (ML) demodulation algorithm is used inconjunction with a maximum ratio combining (MRC) algorithm to decode thedata sequence using the orthogonal transmitting and re-transmittingsignals received by the receiving antennas. Using the two algorithmsmakes the decoding procedure optimal.

In another embodiment of the present invention, the channel is a fastchanging channel. The channel condition at the time of transmitting asignal is completely different from that at the time of re-transmittingthe signal. The different channel conditions creating time diversity,and thus the re-transmitting signal need not be orthogonal to thetransmitting signal. The re-transmission function for a fast changingchannel is defined as

${f\begin{pmatrix}x_{1} \\x_{2}\end{pmatrix}} = {\begin{pmatrix}x_{1} \\x_{2}\end{pmatrix}.}$

Because the transmitting and re-transmitting signals received by thereceiving antennas are not orthogonal to each other, the simplified MLdemodulation algorithm in conjunction with MRC cannot be used to decodethe data sequence. Instead, the traditional Chase combining is used.

In yet another embodiment of the present invention, a virtual channel iscreated between the BTS and the MS so that the optimal decodingprocedure for a fast changing channel can be used. The channel conditionbetween the BTS and the MS is slow changing channel. The virtual channelis represented by the following matrix:

${\begin{matrix}{{\overset{\rightarrow}{h}}_{1}^{\prime}{\overset{\rightarrow}{w}}_{1}^{\prime}} & {{\overset{\rightarrow}{h}}_{1}^{\prime}{\overset{\rightarrow}{w}}_{2}^{\prime}} \\{{\overset{\rightarrow}{h}}_{2}^{\prime}{\overset{\rightarrow}{w}}_{1}^{\prime}} & {{\overset{\rightarrow}{h}}_{2}^{\prime}{\overset{\rightarrow}{w}}_{2}^{\prime}}\end{matrix}}\quad$

(6). The virtual channel is created by using a set of re-transmissionchannel response functions {right arrow over (h)}′₁, {right arrow over(h)}′₂ and a set of re-transmission weighting vectors {right arrow over(w)}′₁, {right arrow over (w)}′₂. By using a re-transmission function

${f\begin{pmatrix}x_{1} \\x_{2}\end{pmatrix}} = {\begin{pmatrix}{- x_{2}^{H}} \\x_{1}^{H}\end{pmatrix}\mspace{14mu} {to}}$

create orthogonal re-transmitting signals, the signals received by theantennas of the antenna array are represented by the followingequations:

r′ ₁ ={right arrow over (h)}′ ₁ ^(H) {right arrow over (w)}′ ₁(−x ₂^(H))+{right arrow over (h)}′ ₁ ^(H) {right arrow over (w)}′ ₂(x ₁ ^(H))(4) and r′ ₂ ={right arrow over (h)}′ ₂ ^(H) {right arrow over (w)}′₁(−x ₂ ^(H))+{right arrow over (h)}′ ₂ ^(H) {right arrow over (w)}′ ₂(x₁ ^(H))  (5),

where r′₁ is the signal received by the first antenna of the antennaarray and r′₂ is the signal received by the second antenna.

The beamforming weighting vectors {right arrow over (w)}′₁ and {rightarrow over (w)}′₂ are computed in accordance with the requirement thatone of the two following conditions must be met. The first condition is{right arrow over (h)}′₁{right arrow over (w)}′₁={right arrow over(h)}₁{right arrow over (w)}₁, {right arrow over (h)}′₁{right arrow over(w)}′₂={right arrow over (h)}₁{right arrow over (w)}₂, {right arrow over(h)}′₂{right arrow over (w)}′₁={right arrow over (h)}₂{right arrow over(w)}₁, and {right arrow over (h)}′₂{right arrow over (w)}′₂={right arrowover (h)}₂{right arrow over (w)}₂. The second condition is {right arrowover (h)}′₁{right arrow over (w)}′₁≈{right arrow over (h)}₁{right arrowover (w)}₁, {right arrow over (h)}′₁{right arrow over (w)}′₂≈{rightarrow over (h)}₁{right arrow over (w)}₂, {right arrow over (h)}′₂{rightarrow over (w)}′₁≈{right arrow over (h)}₂{right arrow over (w)}₁. If thefirst condition is met, the simplified ML demodulation algorithm can beused in conjunction with MRC to decode the data sequence. Using the twoalgorithms makes the decoding procedure optimal while maintains timediversity. However, if the second condition is met the performance ofthe wireless network with the embodiment described above will experiencesome degree of degradation.

The above illustration provides many different embodiments orembodiments for implementing different features of the invention.Specific embodiments of components and processes are described to helpclarify the invention. These are, of course, merely embodiments and arenot intended to limit the invention from that described in the claims.

Although the invention is illustrated and described herein as embodiedin one or more specific examples, it is nevertheless not intended to belimited to the details shown, since various modifications and structuralchanges may be made therein without departing from the spirit of theinvention and within the scope and range of equivalents of the claims.Accordingly, it is appropriate that the appended claims be construedbroadly and in a manner consistent with the scope of the invention, asset forth in the following claims.

1. A method for retransmitting a data sequence according to channelcondition in a wireless communications network, the method comprising:transmitting a first transmitting signal representing the data sequenceusing a first set of beamforming weighting vectors generated accordingto a first channel condition; receiving a request for re-transmittingthe data sequence, wherein the first transmitting signal suffers fromunrecoverable errors; computing a re-transmission function according toa comparison between the a second channel condition and the firstchannel condition; and transmitting a second transmitting signalgenerated from the first transmitting signal by using there-transmission function, wherein the data sequence is obtained bydemodulating and decoding both the first and second transmitting signals2. The method of claim 1, wherein the first transmitting signalrepresenting the data sequence is generated from a transmission unit ofa sequence of radio symbols.
 3. The method of claim 2, wherein thesequence of radio symbols comprises Orthogonal Frequency-DivisionMultiplexing (OFDM) symbols.
 4. The method of claim 2, wherein the firsttransmitting signal S is generated according to the following equation:{right arrow over (s)}=x₁{right arrow over (w)}₁+x₂{right arrow over(w)}₂, where (x_(l), x₂) is a transmission unit of a sequence of radiosymbols; and {right arrow over (w)}₁ and {right arrow over (w)}₂ are aset of beamforming weighting vectors.
 5. The method of claim 1, whereinthe comparison between the first and second channel condition representsthat of a quasi-static channel.
 6. The method of claim 5, wherein there-transmission function of the quasi-static channel is defined as${{f\begin{pmatrix}x_{1} \\x_{2}\end{pmatrix}} = \begin{pmatrix}{- x_{2}^{H}} \\x_{1}^{H}\end{pmatrix}},$ where (x₁, x₂) is a transmission unit of a sequence ofradio symbols.
 7. The method of claim 1, wherein the second transmittingsignal {right arrow over (s)}′ is generated according to the followingequation: {right arrow over (s)}′=(−x₂ ^(H)){right arrow over (w)}₁+(x₁^(H)){right arrow over (w)}₂, where (x₁, x₂) is a transmission unit of asequence of radio symbols; {right arrow over (w)}₁ and {right arrow over(w)}₂ are beamforming weighting vectors; and ( . . . )^(H) is aHermitian operator.
 8. The method of claim 7, wherein the secondtransmitting signal is orthogonal to the first transmitting signal. 9.The method of claim 8, wherein the first and the second transmittingsignals form a combined signal.
 10. The method of claim 9, wherein thecombined signal is demodulated and decoded by using a simplified maximumlikelihood (ML) demodulation algorithm in conjunction with a maximumratio combining (MRC) algorithm.
 11. The method of claim 1, wherein thesecond channel condition represents that of a fast changing channel. 12.The method of claim 11, wherein the re-transmission function of the fastchanging channel is defined as ${{f\begin{pmatrix}x_{1} \\x_{2}\end{pmatrix}} = \begin{pmatrix}x_{1} \\x_{2}\end{pmatrix}},$ where (x₁, x₂) is a transmission unit of a sequence ofradio symbols.
 13. The method of claim 12, wherein the secondtransmitting signal is the same as the first one.
 14. The method ofclaim 13, wherein the first and the second transmitting signals form acombined signal.
 15. The method of claim 14, wherein the combined signalis demodulated and decoded by using the Chase combining algorithm.
 16. Amethod for retransmitting a data sequence according to channel conditionin a wireless communications network, the method comprising:transmitting a first transmitting signal, generated from a transmissionunit of a sequence of radio symbols comprising OrthogonalFrequency-Division Multiplexing (OFDM) symbols representing the datasequence, using a first set of beamforming weighting vectors generatedaccording to channel condition; receiving a request for re-transmittingthe data sequence, wherein the first transmitting signal suffers fromunrecoverable errors; computing a re-transmission function for aquasi-static channel; and transmitting a second transmitting signal,which is orthogonal to the first one, generated by using there-transmission function that is created for the quasi-static channel,wherein the data sequence is demodulated and decoded using a combinedsignal formed by the first and the second transmitting signals.
 17. Themethod of claim 16, wherein the first transmitting signal {right arrowover (s)} is generated according to the following equation: {right arrowover (s)}=x₁{right arrow over (w)}₁+x₂{right arrow over (w)}₂, where(x₁, x₂) is a transmission unit of a sequence of radio symbols; and{right arrow over (w)}₁ and {right arrow over (w)}₂ are beamformingweighting vectors.
 18. The method of claim 16, wherein there-transmission function of the quasi-static channel is defined as${{f\begin{pmatrix}x_{1} \\x_{2}\end{pmatrix}} = \begin{pmatrix}{- x_{2}^{H}} \\x_{1}^{H}\end{pmatrix}},$ where (x₁, x₂) is a transmission unit of a sequence ofradio symbols.
 19. The method of claim 16, wherein the secondtransmitting signal {right arrow over (s)}′ is generated according tothe following equation: {right arrow over (s)}′=(−x₂ ^(H)){right arrowover (w)}₁+(x₁ ^(H)){right arrow over (w)}₂, where (x₁, x₂) is atransmission unit of a sequence of radio symbols; {right arrow over(w)}₁ and {right arrow over (w)}₂ are beamforming weighting vectors; and( . . . )^(H) is a Hermitian operator.
 20. The method of claim 16,wherein the combined signal is demodulated and decoded by using asimplified maximum likelihood (ML) demodulation algorithm in conjunctionwith a maximum ratio combining (MRC) algorithm.
 21. A method forretransmitting a data sequence according to channel condition in awireless communications network, the method comprising: transmitting afirst transmitting signal, generated from a transmission unit of asequence of radio symbols comprising of Orthogonal Frequency-DivisionMultiplexing (OFDM) symbols representing the data sequence, using afirst set of beamforming weighting vectors generated according to achannel condition; receiving a request for re-transmitting the datasequence, wherein the first transmitting signal suffers fromunrecoverable errors; computing a re-transmission function for a fastchanging channel; and transmitting a second transmitting signal,generated for the fast changing channel, which is the same as the firsttransmitting signal, wherein the data sequence is demodulated anddecoded with the Chase combining algorithm, using the first and thesecond transmitting signals.
 22. The method of claim 21, wherein thefirst transmitting signal {right arrow over (s)} is generated accordingto the following equation: {right arrow over (s)}=x₁{right arrow over(w)}₁+x₂{right arrow over (w)}₂, where (x₁, x₂) is a transmission unitof a sequence of radio symbols, and {right arrow over (w)}₁ and {rightarrow over (w)}₂ is a set of beamforming weighting vectors.
 23. Themethod of claim 11, wherein the re-transmission function for the fastchanging channel is defined as ${{f\begin{pmatrix}x_{1} \\x_{2}\end{pmatrix}} = \begin{pmatrix}x_{1} \\x_{2}\end{pmatrix}},$ where (x₁, x₂) is a transmission unit of a sequence ofradio symbols.
 24. A method for retransmitting a data sequence accordingto channel condition in a wireless communications network, the methodcomprising: transmitting a first transmitting signal, generated from atransmission unit of a sequence of radio symbols comprising OrthogonalFrequency-Division Multiplexing (OFDM) symbols representing the datasequence, using a first set of beamforming weighting vectors generatedaccording to channel condition; receiving a request for re-transmittingthe data sequence, wherein the first transmitting signal suffers fromunrecoverable errors; computing a re-transmission function for a slowchanging channel; and transmitting a second transmitting signal, whichis orthogonal to the first one, generated by using the re-transmissionfunction that is created for the quasi-static channel, wherein the datasequence is demodulated and decoded using a combined signal formed bythe first and the second transmitting signals.
 25. The method of claim24, wherein the re-transmission function of the slow changing channel isdefined as ${{f\begin{pmatrix}x_{1} \\x_{2}\end{pmatrix}} = \begin{pmatrix}{- x_{2}^{H}} \\x_{1}^{H}\end{pmatrix}},$ where (x₁, x₂) is a transmission unit of a sequence ofradio symbols.
 26. The method of claim 24, wherein virtual channelsrepresented by the following matrix: ${\begin{matrix}{{\overset{\rightarrow}{h}}_{1}^{\prime}{\overset{\rightarrow}{w}}_{1}^{\prime}} & {{\overset{\rightarrow}{h}}_{1}^{\prime}{\overset{\rightarrow}{w}}_{2}^{\prime}} \\{{\overset{\rightarrow}{h}}_{2}^{\prime}{\overset{\rightarrow}{w}}_{1}^{\prime}} & {{\overset{\rightarrow}{h}}_{2}^{\prime}{\overset{\rightarrow}{w}}_{2}^{\prime}}\end{matrix}}\quad$ are created by using the re-transmission functionand a set of re-transmission weighting vectors {right arrow over (w)}′₁,{right arrow over (w)}′₂.
 27. The method of claim 26, wherein the set ofre-transmission weighting vector {right arrow over (w)}′₁, {right arrowover (w)}′₂ is computed in accordance with the following requirement,{right arrow over (h)}′₁{right arrow over (w)}′₁={right arrow over(h)}₁{right arrow over (w)}₁, {right arrow over (h)}′₁{right arrow over(w)}′₂={right arrow over (h)}₁{right arrow over (w)}₂, {right arrow over(h)}′₂{right arrow over (w)}′₁={right arrow over (h)}₂{right arrow over(w)}₁, and {right arrow over (h)}′₂{right arrow over (w)}′₂={right arrowover (h)}₂{right arrow over (w)}₂.
 28. The method of claim 26, whereinthe set of re-transmission weighting vector {right arrow over (w)}′₁,{right arrow over (w)}′₂ is computed in accordance with the followingrequirement, {right arrow over (h)}′₁{right arrow over (w)}′₁={rightarrow over (h)}₁{right arrow over (w)}₁, {right arrow over (h)}′₁{rightarrow over (w)}′₁≈{right arrow over (h)}₁{right arrow over (w)}₁, {rightarrow over (h)}′₁{right arrow over (w)}′₂≈{right arrow over (h)}₁{rightarrow over (w)}₂, {right arrow over (h)}′₂{right arrow over(w)}′₁≈{right arrow over (h)}₂{right arrow over (w)}₁.
 29. The method ofclaim 24, wherein the first and the second transmitting signals form acombined signal.
 30. The method of claim 29, wherein the combined signalis demodulated and decoded by using a simplified maximum likelihood (ML)demodulation algorithm in conjunction with a maximum ratio combining(MRC) algorithm.